This application claims the priority of German Application No. 199 25 271.8 filed Jun. 2, 1999, which is incorporated herein by reference.
This invention relates to a draw unit for a spinning preparation machine, particularly a regulated draw frame for cotton or chemical fibers and the like. The draw unit has at least two consecutively arranged roll pairs each having a driven roll provided with its own electromotor as well as an electronic control and regulating device to which the electromotors are connected. A respective incremental rotational displacement sensor is coupled with the driven rolls.
In a known draw unit, as disclosed in German patent document No. 196 44 560, to which corresponds U.S. Pat. No. 5,991,977, a device for preventing a reverse rotation during standstill is associated with the lower input roll.
It is an object of the invention to provide an improved method of operating a draw unit of the above-outlined type in which, upon reverse rotation of the rolls during standstill the predetermined extent of draw of the sliver bundle is preserved in a simple manner.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the method of operating a sliver draw unit of a spinning preparation machine includes the following steps: applying, by incremental rotary displacement sensors, signals to an electronic control and regulating device; determining from the signals the angular position and/or the rotary direction of at least one of the roll pairs of the draw unit during operation and standstill; and rotating at least one of the roll pairs into a predetermined angular position by controlling the electric motor driving that roll pair.
The incremental rotary displacement sensors are associated with the rolls at the input and with the rolls at the output of the draw unit whereby a highly responsive monitoring of the rpm and the direction of rotation of the rolls is possible to thereby permit suitable countermeasures or intervention. In particular, an undesired reverse rotation of the intake roll or the intake and mid roll is compensated for by a slight forward rotation during standstill of the outlet rolls so that locations of reduced thickness or even ruptures in the sliver are avoided. The predetermined draft of the sliver is maintained during standstill even in case of a possible reverse rotation of the input roll. The forward rotation of the intake roll causes the sliver to sag slightly which is harmless. Then, in case of a reverse rotation, the sagging region is simply pulled straight without causing a reduction in the sliver thickness or a rupture. It is a further particular advantage of the invention that based on the known position of the rolls in operation, during standstill and after switching off the machine, particularly the drive motors, the overhanging sliver is first pulled taut or set in a simple manner with the aid of the controlled motors. Thereafter the rolls are accelerated to the operational rpm, and the sliver is accelerated to the operational speed while maintaining the predetermined draft (despite an undesired reverse rotation of the intake roll that may occur). The invention eliminates the need for a mechanical or an electromechanical lock to prevent a reverse roll rotation.
The invention has the following additional advantageous features:
The electronic control and regulating device may set the sliver in or upstream of the draw unit to a predetermined draft by a suitable rpm control of the drive motor or drive motors.
During standstill of the output roll, the input roll or the input roll and the mid roll continue to rotate forward to a small predetermined extent.
The extent of the forward rotation equals at least the extent of the reverse rotation of the rolls caused by a relaxation of the sliver during standstill.
After reaching the predetermined forward rotation, the input roll or the input roll and the mid roll is placed in a standstill state.
The input rolls or the input rolls and the mid rolls and the output rolls are braked to assume standstill and subsequently the input roll or the input roll and the mid roll are slightly accelerated in the working (forward) direction.
When the machine is stopped and after the standstill state is reached, the principal electric motor (driving the output roll pair) is turned off, and then the regulating electric motor (driving the input roll pair or the input roll pair and the mid roll pair) is switched on and is subsequently switched off.
Upon stopping the machine the rpm of the principal motor is reduced to zero from the operational rpm and the rpm of the regulating motor is first reduced to a value greater than zero and thereafter is reduced to zero.
Upon turning on the motors the roll rpm""s are set to correspond to a predetermined draft of the sliver.
Upon turning on the motors, deviations from the predetermined positions (desired values) are compensated for.
The output rolls are brought into a predetermined position.
The input rolls or the input rolls and the mid rolls are brought into a predetermined position.
After setting the rolls to predetermined (desired) positions, the rolls are accelerated to the operating rpm.
The predetermined rotary displacement is between zero and 4 mm.
The predetermined rotary displacement is zero; the reverse rotation may be eliminated by a slight acceleration in the working direction.
The predetermined rotary displacement is zero; the reverse rotation corresponds to the small rotary displacement in the working direction.
The displacement of the sliver in the working direction is approximately between 0.1 and 4 mm.
The sliver is accelerated in the principal draw field during standstill of the output rolls by a small rotation of the input roll or the input roll and the mid roll.